According to national and regional standards, the rated active or nominal power of an electrical machine is indicated as a function of the operating type. In order to avoid permanent damage to components of the electrical machine, the electrical machines are configured so that the indicated rated power will not be exceeded or the power is restricted with the aid of suitable monitoring of voltages and/or currents and/or temperatures, as the case may be. This power restriction provided for component protection is frequently also referred to as “de-rating”.
In various applications of an electrical machine, such as in the automotive sector, it is believed that special requirements and marginal conditions must be taken into account, which results in the goal of setting up the electrical machine so that the highest possible output density is achieved at the smallest size possible.
It is also believed to be understood to use electrical machines as drive units in vehicles. In cases where an electrical machine is used as a drive unit in an electric or hybrid vehicle, it must also be taken into account in the configuration of the electrical machine that the operational readiness of the vehicle has to be ensured at all times; furthermore, a power desired by the driver or a torque that satisfies the driver demand must be able to be provided without fail. In such an application, in particular, the goal of the design and the operating strategy of the electrical machine thus must consist of minimizing the occurrence of an automatic power restriction for component protection purposes.
As a rule, electrical machines in the form of polyphase machines, especially polyphase synchronous machines, which are operated in combination with pulse-controlled inverters, often also simply called inverters, may be used as drives in hybrid and electric vehicles. The electrical machines are selectively operated in motor operating mode or generator operating mode. In motor operation, the electrical machine generates a drive torque which, when used in a hybrid vehicle, assists an internal combustion engine, such as in an acceleration phase. In generator operation, the electrical machine generates electrical energy, which is stored in an energy store, for example a traction battery.
Depending on the driver's intentions and possibly additional operating parameters of the vehicle, a working point of the electrical machine is specified, which is defined by the torque and the engine speed of the electrical machine. In the case of electrical machines having separate excitation, the working point is adjustable by appropriate input of the excitation current and the stator or phase current. It is known and common to regulate an electrical machine having separate excitation in a predefined working point using an optimal efficiency factor, which means that the excitation current or the stator currents is specified so that the desired working point is reached at optimum efficiency. For example, from the German patent DE 198 49 889 A1, a method is discussed for controlling an electrical machine in a manner that is optimized with regard to power and efficiency; in this method, at least three control ranges are formed in which the control of an excitation current and of phase currents takes place according to different criteria, the excitation current and the stator currents being freely selectable for all rotational speeds and desired powers with regard to amount and phase.